Osmosis

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Glossary

Key terms and definitions.

diffusion: Passive movement of molecules down a concentration gradient.

flaccid: State of a plant cell in a hypertonic solution.

hypertonic: Solution that has a higher concentration of solute than the cell.

hypotonic: Solution that has a lower concentration of solute than the cell.

isotonic: Solution that has an equal concentration of solute compared with the cell.

lysis: Animal cell bursting in a hypotonic solution.

osmosis: Movement of water molecules from an area of high concentration to an area of low concentration through a semipermeable membrane.

passive transport: Transport of a substance across a cell membrane by diffusion; Energy is not required.

semi-permeable: Ability of cell membranes to allow some substances to pass it but not others.

solute: A substance dissolved in water.

tonicity: Difference in relative concentration of solutes of two solutions, which determines the direction of osmosis.

turgidity: Pressure created by water pushing up against the cell wall

turgor pressure: The pressure that the content of the cell exerts on the cell membrane, pushing it against the cell wall

Osmosis is Passive

OSMOSIS is the movement of water across a SEMIPEREABLE membrane, from an area of high water concentration to an area of low water concentration.

As with simple DIFFUSION and FACILITATED DIFFUSION, the water molecules are moving down the concentration gradient. Osmosis does not require any energy from RESPIRATION to move water molecules down a concentration gradient, therefore it is a type of PASSIVE TRANSPORT.

For example: Osmosis in root hair cells

In plants, water is taken up from the soil by special cells on a plant’s roots called root hair cells.

For OSMOSIS to happen, the soil must have a higher concentration of water than inside the root hair cell in order for water to move into the root cell’s SEMIPERMEABLE membrane and into the root cell.

This osmosis happens after rainfall, when there is a higher concentration of water in the soil than inside a plant’s cells. The water moves down the concentration gradient and into the plant, through the cell membrane of the root hair cells. All of this happens passively, meaning it doesn’t require energy.

Video: Osmosis and Water Potential (Amoeba Sisters). More in-depth than you need to know, use as a general overview. This has a worksheet associated with it. Let me know if you'd like to have access to this worksheet.

Tonicity

There are many different substances that can be dissolved in water. These dissolved substances are called SOLUTES.

The concentration of these solutes on either side of a cell’s semipermeable membrane will affect how water moves into our out of the cell by OSMOSIS. The measure of the concentration of these solutes on either side of the membrane is called TONICITY.

HYPERTONIC Solution

Hypertonic solutions are very concentrated solutions. E.g. very salty or sugar water.

The water concentration is higher in the cell, so the water is going to want to move out of the cell and into the extracellular environment.

The cell shrinks in size and becomes FLACCID (plant) or PLASMOLYSED (animal).

HYPOTONIC Solutions

Hypotonic solutions are very dilute solutions. E.g. pure water.

The water concentration is higher in the extracellular environment, so the water is going to want to move into the cell.

The cell grows in size and becomes TURGID (plant). The animal cell keeps growing until the cell membrane ruptures (LYSIS).

ISOTONIC Solutions

Isotonic solutions are neither concentrated nor dilute. E.g. saline.

The water concentration is the same inside the cell and in the extracellular environment, so there is no net movement of water.

The cell does NOT change size.

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